Hydraulic winches are used in a variety of operations to perform lifting, lowering, or towing operations. For example, pipelayers are specialized machines that use winches to install large, heavy lengths of conduit into or above ground. In some applications, the conduits are used to carry oil and gas from remote well locations over vast distances to a receiving station or refinery. The use of a pipeline avoids transportation costs for shipping, trucking, or otherwise moving the oil or gas. In addition to petroleum pipelines, pipelayers can also be used to install piping for other materials, or for installing drain tile, culverts, or other irrigation and drainage structure. In these applications, winches are used to lift, position, and/or lower heavy loads.
To provide sufficient power for heavy loads, many winches are hydraulically driven. Hydraulic winches typically include a drum about which is wound a cable. The drum is coupled to a hydraulic motor, which in turn is hydraulically coupled to a hydraulic pump. The hydraulic pump may be operatively coupled to an engine of the machine or other prime mover. A brake may also be provided that may normally engage and hold the drum in place, but may be disengaged to permit drum rotation. An operator interface, such as a lever, may be coupled to the pump and brake through a controller to enable a user to operate the winch.
Conventional control systems for hydraulic winches do not permit relative fine control of the load, particularly in lowering operations. Some conventional systems employ a counterbalance valve to address this issue, but the counterbalance valve adds a considerable amount of heat to the system during use, thereby necessitating additional flushing of the hydraulic circuit to maintain acceptable operating temperatures. If the counterbalance valve is removed from these systems, however, the brake is released simultaneously with operation of the pump, causing drum speed to be too aggressive and the load to overshoot the desired position before pump operation can be adjusted.
U.S. Pat. No. 7,166,061 to Shimomura et al. discloses a control device for a hydraulic winch having a hydraulic motor and a brake coupled to the winch. The control device uses a load pressure on the hydraulic motor and external command signals to adjust a capacity of the motor. When the control device executes an automatic shutoff, during which the brake is activated to hold the winch in place, it also increases the motor capacity sufficient to maintain the load, thereby to improve response time of the winch when the automatic shutoff is released. While response time may be improved, Shimomura does not address fine control of the winch when it begins to move from a halt position, particularly since the Shimomura control device increases motor capacity to match the load.